Organometallic Reagents for Synthetic Purposes: Tellurium
Nicola Petragnani*, and Wai-Ling Lo
Instituto de Química, Universidade de São Paulo, Av. Lineu Prestes, 748, C.P. 26077, 05599-970 São Paulo, SP - Brazil
Received: December 16, 1997
As principais aplicações sintéticas de reagentes contendo telúrio foram reunidas e estão apresentadas a seguir.
An overview of the most important applications of tellurium compounds in Organic Synthesis is presented.
Keywords:tellurides, organotellurium compounds
Introduction
About a quarter of a century ago, Tellurium was con-sidered almost as an exotic element and was treated with certain diffidence by the most serious chemists. Then the explosive development of Selenium chemistry called atten-tion to the potentiality of Tellurium analogues, resulting in studies on preparation methods for inorganic and organic tellurium compounds and particularly on their applications in Organic Synthesis. In the mid 50’s, H. Rheinboldt pub-lished the first review (about 50 pages) on selenium and tellurium compounds in organic chemistry1a, then in 1990 K. Irgolic dedicated as much as 1000 pages in a review on tellurium compounds1b. The impressive number of publi-cations on this subject during the last few years shows that Tellurium already plays a new role as a powerful chemical tool. Herein we present an overview of the most important applications of Tellurium compounds in Organic Synthe-sis.
Reduction
A large variety of organic substrates can be reduced by tellurium reagents, for instance carbonyl compounds and their α,β-conjugated analogues, aryl alkenes and alkynes, nitro compounds and other nitrogenated derivatives. Com-pounds bearing heteroatoms can be submitted to reductive cleavage of the carbon-heteroatom bond. The most com-monly used reagents are H2Te, NaHTe, Na2Te, PhTeH and
PhTeNa. They offer some advantages over conventional reagents such as milder reaction conditions, better
regiose-lectivity, in situ preparation of the reducing reagent, some-times in catalytic conditions, and the recovery of tellurium in the elemental form or as diphenyl ditelluride.
Reduction of carbonyl compounds
Aldehydes and ketones can be reduced to the corre-sponding primary and secondary alcohols by H2Te and
PhTeH. The former is prepared by in situ hydrolysis of Al2Te3 and the latter by treatment of PhTeLi with
CF3CO2H or methanolysis of PhTeSiMe32-4. An additional
method employs the Bu2Te/TiCl4 system to generate Ti
(III) species, which is the actual reducing agent5.
Selective reduction of the C=C bond of α,β-conjugated carbonyl compounds (aldehydes, ketones, esters, acids and amides) is achieved by treatment with H2Te,2 PhTeH3,4 and
NaHTe.3,6 The latter is prepared by treatment of elemental tellurium with NaBH4 in ethanol under controlled
acidifi-cation with acetic acid.
Reduction of conjugated aryl alkenes and alkynes
C=C and C≡C bonds conjugated to aromatic groups are reduced to the corresponding saturated bonds by treatment with PhTeH3,4 or NaHTe3.
Reduction of nitro compounds and other nitrogenated derivatives
Nitroarenes give rise to different nitrogen compounds such as anilines2-5,7, azo8 and azoxy7d,8b compounds and aryl hydroxylamines9 depending on the reducing agent and experimental conditions, while imines3,10 and enamines10a
Review
are reduced to the corresponding amines by H2Te, NaHTe
and PhTeH. An interesting and useful application is the one-step reductive alkylation of amines with carbonyl com-pounds10a,11 (Eq. 1).
Reduction of oxiranes
Oxiranes can either be directly deoxygenated to alkenes by (EtO)2P(O)TeM (M = Na, Li)12 in a stereoselective
manner or via a two-step methodology, by reduction with NaHTe to a telluro alcohol (1), then a tosylation-elimina-tion sequence furnishes an unstable epitelluride intermedi-ate (2) which easily detellurates to an alkene13. Telluro alcohol (1) can also undergo reductive detelluration with nickel boride affording the corresponding alcohol, which in turn can be further oxidized to ketones13 (Scheme 1).
α,β-Epoxyketones are reduced to the corresponding
β-hydroxy ketones by NaHTe14. Epoxides bearing leaving groups in a suitable position are converted to allylic alco-hols via formation of an epitelluride intermediate15. This methodology, combined with the Sharpless kinetic resolu-tion of secondary allylic alcohols, provides a useful proce-dure for the conversion of racemic alcohols to pure single enantiomers16 (Scheme 2).
Debromination of vicinal dibromides
This reaction proceeds via a typical anti-E2 elimination
by usage of reagents such as diaryl tellurides17a and ditel-lurides17b, NaHTe17c, Na2Te17d, (EtO)2P(O)TeNa17e,
2-ThTeNa17f and (Ph3Sn)2Te17g. The advantages over the
conventional methods are milder experimental conditions, higher yields, lack of side reactions and inertness of many functional groups towards the tellurium reagents. Several procedures can also be carried out using catalytic quantites of tellurium compound in the presence of a suitable redu-cing reagent, for instance potassium metabisulfite, for the regeneration of tellurium reagent17h (Scheme 3).
Reductive cleavage of carbon-heteroatom bonds
Several tellurium reagents promote the title reductive cleavage on α-halo ketones, whereas thienyl tellurolate can
J. Braz. Chem. Soc.
H R1
R
R2 O
R3 = H CrO3, Py
H R3
R1
R OH
R2 Ni3B2
2 1
NaHTe O
R R1
R2 R3
(EtO)2P(O)TeM
M = Na, Li
(EtO)2P(O)OM+ Te
R R1
R2 R3 +
HTe R3
R1
R OH
R2
TsCl, Py
-TsOH R R1
R2 R3 Te
- Te
Scheme 1.
single enantiomer H+ - Te
R
R1 O
-Te R
R1 O
Te -Ms
Te2-R
R1 O
O (b) Na2Te
(a) Ms2O, Py
R
R1 OH
+ R
R1 OH
O
epoxidation Sharpless R
R1 OH
Scheme 2.
(1) R
R1
NHR2
NaHTe H2Te or
R2NH2 + R
R1 O
R
R1 K2S2O5
Ar2TeBr2
Ar2Te
R
Br
Br
R1
also remove other heteroatom substituted leaving groups18 (Eq. 2).
Tellurium Promoted Formation of Anionic
Species and Reaction with Electrophiles
Nucleophilic attack of Na2Te at the halogen on α-halo
esters or nitriles leads to the corresponding carbanions which react with aldehydes giving α,β-unsaturated com-pounds according to Reformatsky-type reactions19. The same tellurium reagent undergoes reaction with Ph2S2 or
ArSO2Cl affording thiolate and sulfinate anions through an
electron transfer from the telluride ion. Further in situ reaction with α-halo carbonyl compounds or alkyl halides leads to α-thio carbonyl compounds and sulfones20 (Sche-me 4).
Bu2Te promotes in situ generation of triphenyl
methylene phosphorane from the parent phosphonium io-dide for further Wittig methylenation reactions. The pecu-liarity of this methodology is the formation of phosphorane in neutral conditions21 (Scheme 5).
Deprotection of Organic Functionalities
Anionic tellurium species offer great possibilities for the regeneration of protected functional groups, the reac-tions proceeding under mild condireac-tions in non-hydrolytic media. Carboxylic acids protected as alkyl or benzyl es-ters22a, as well as phenacyl22c, allyl22d and β-halo ethyl
carboxylates22b,e,f can be regenerated by NaHTe, Na 2Te
and Na2Te2. Phenols can be regenerated from aryl
carboxy-lates23a, halo acetates22e or carbonates23b by NaHTe and
from phenyl allyl ethers22d by Na
2Te. Amines protected as
trichloro-t-butyl carbamates can be regenerated by 2-ThTeNa24 (Scheme 6).
Oxidation
An2TeO is an easily prepared, stable crystalline
com-pound which has been successfully employed as a mild and selective oxidizing reagent, with the advantage of possible regeneration from the corresponding telluride after the oxidative process. (ArTeO)2O has recently been introduced
as an oxidizing reagent, similarly to the former oxidant. Typical oxidations25 are: thiols to disulfides, hidroquinones
to quinones, diols and benzyl alcohols to aldehydes, acyl hydrazines to acyl hydrazides, N-phenyl hydroxylamine to nitrosobenzene, benzophenone hydrazone to diphenyl dia-zomethane and phenyl isothiocyanate to diphenylurea. One important transformation is the oxidation of thio and seleno carbonyl compounds to the oxo-analogues. These oxida-tions can also be accomplished by a catalytic procedure (Scheme 7).
By treatment with (PhTeO)2O in boiling AcOH under
sulfuric acid catalysis, alkenes give rise to syn -diacetoxy-lated products26. Other methodology employs TeO2/LiBr
or TeCl4/LiOAc in AcOH. Aryl alkenes can also be
di-methoxylated by treatment with Ph2Te2 and t-butyl
perox-ide in MeOH and sulfuric acid27.
Telluro Cyclofunctionalization
Addition of an electrophilic tellurium reagent, for in-stance ArTeCl3, to alkenes C=C bond bearing a
nucleo-philic moiety at a suitable position, followed by intramolecular trapping of the telluronium intermediate furnishes cyclic structures where the tellurium moiety lies outside the newly ring. Such reactions are known as telluro-cyclofunctionalization and they can be either lactoniza-tion28 or etherification29 (Scheme 8).
R X
O
R1 R2
[ Red. ]
R H
O
R1 R2
(2)
X = Cl, Br, I ; [Red.] = NaHTe, (EtO)2P(O)TeNa, 2-ThTeLi (Na), (Ph3Sn)2Te/KF
X = OAc, OMs, SPh; [Red.] = 2-ThTeLi (Na)
ArSO2R RX
ArSO2Na
- Te (EtO)2P(O)TeNa
Na2Te or ArSO2Cl
R SPh
O
2PhSNa - Te
Na2Te
PhSSPh R
X O
Scheme 4.
R H
RCHO
Ph3P CH2
- Bu2TeI2
Bu2Te
+
Ph3P I ]
I-Scheme 5.
R12C CCl2 +
R2NH - CO2
(2ThTe)
-R2N O
Cl O
Cl Cl
R1 R1
2-ThTeNa
R2N O
Cl O R1 R1
Cl Cl
Other useful reagents for these cyclizations are: PhTeOSO2C6H4NO2-430a for lactonizations and
etherifica-tions, and ArTe(O)OAc30b, TeO2/AcOH/LiCl30c and
TeO2/HCl/MeOH30d for etherifications. Similar reactions
are also well known with the selenium analogs. Reductive removal of the tellurium moiety from the cyclic products can be achieved by further reaction with Bu3SnH31.
Replacement of Tellurium Moiety with
Other Atoms
Replacement by halogens: halogenodetelluration
ArTeCl3 and Ar2TeCl2 bearing a para
-electron-donat-ing group on the aromatic r-electron-donat-ing undergo Te/I exchange in the presence of fluoride anions through an electrophilic mechanism32a. A similar reaction has also been carried out on vinylic tellurium trichlorides (3) with I2 or NBS. Since
compounds 3 are easily prepared from phenylacetylenes,
this two-step sequence offers a simple methodology for the
syn-iodo- or bromo-chlorination of alkynes32b (Scheme 9).
Ipso-substitution (or α-elimination) on tellurium (IV) halides, i.e. selective transfer of halogen from tellurium to the carbon moiety33, can be performed on suitable sub-strates such as RTeCl3 (R= alkyl, vinyl, aryl), Ar2TeCl2 and
PhTe(Br)2R (R= alkyl) by oxidative, photolytic or
pyro-lytic procedures depending on the tellurium substrates. The latter procedure is useful for homologation of alkyl halides (Scheme 10). Other halogenodetellurations can also be accomplished by reaction of α-dichloro telluro ketones34a
and acetylenic tellurides34b with halogens. Replacement by alkoxy group
Alkyl phenyl telluroxides are easily obtained by oxida-tion of parent tellurides with mCPBA or by alkaline hy-drolysis of the corresponding dibromides. Treatment of telluroxides with excess mCPBA or CF3CO3H in alcohols
leads to detelluration and formation of alkyl ethers. Alkyl phenyl tellurones, obtained by oxidation of telluroxides with NaIO4 give similar results under the same
condi-tions35a (Scheme 11).
Phenyl migration35a has been reported on structures
bearing a phenyl group vicinal to the tellurium moiety. This
J. Braz. Chem. Soc.
Ipso-substitution
Te X X
X
Te X X
X
X
Homologation of alkyl halides
RX
(a) PhTeCH2Li
(b) X2
RCH2TePh
X X RCH2X
heating, reduced pressure
- PhTeX
Scheme 10. +
Ar Te Cl Cl Cl
Nu
ArTeCl3
Nu
Tellurolactonization :
O Te Cl Cl
O Ar
Nu = CO2H
Telluroetherification :
O Te Cl Cl Ar
n
Nu = OH Nu
Te Cl Cl Ar
Scheme 8.
3 TeCl4 Ph
Cl
R
TeCl3
Ph
Cl
R
I
Ph Cl
R Br I2
NBS
AlCl3
Ph R
Scheme 9. Y
[ Oxid. ]
Y = S, Se
[Oxid.] = MeO
2
TeO ; (ArTeO)2O
O
Catalytic process :
Ar2TeO Ar2TeBr2 Ar2Te
OH-/H2O R2C=O
R2C=S
(Cl2BrC)2
Cl2C=CCl2
Scheme 7.
O
R R1
or CF3CO2H, H2O2, R1OH mCPBA, R1OH
RTe(O)Ph
NaIO4
RTePh
O O
mCPBA, MeOH
O
R Me
methodology has been used to prepare α-aryl propanoic acids, which are important anti-inflamatory pharmaceuti-cals, from aryl ethyl ketones35b. Ring contractions of 1-tel-lurium-2-alkoxy-cyclohexanes has also been accomplished under similar detellurative methoxylation conditions35a.
Replacement by hydrogen
Phenyl alkyl tellurides undergo mild reductions with Ph3SnH furnishing the corresponding alkanes36. Since the
starting tellurides are easily prepared from nucleophilic substitution of alkyl halides with tellurolate anions, the overall sequence offers a mild reduction methodology for alkyl halides.
Detelluration with C-C Bond Formation
Detellurative coupling reaction
The extrusion of tellurium from organotellurium com-pounds leading to coupled products can be achieved by Raney-Ni37a,b, Pd (0)37c or Li2PdCl437d (Scheme 12).
Detellurative cross-coupling reaction
Alkenes bearing electron-withdrawing groups undergo arylation or vinylation by treatment with Ar2TeCl2,
ArTeCl3, Ar2Te or vinylic tellurides38 in the presence of
PdCl2 or Pd(OAc)2 (Scheme 13). These cross-coupling
reactions can be carried out using catalytic amounts of Pd (II) salts in the presence of a suitable oxidant such as
t-BuOOH, CuCl2 or AgOAc. Under Ni (II) or Co (II)
phosphine complex catalysis, some organotellurides react with Grignard reagents furnishing both cross-coupled and homo-coupled products39 (Scheme 13).
Detellurative carbonylation
Pd (II) also promotes detellurative carbonylations40 of alkyl, vinyl, alkynyl and phenyl tellurides with CO/MeOH/Et3N giving methyl carboxylates. While
per-formed in the presence of CuCl2 as oxidant, only catalytical
amount of Pd (II) is needed. The same methodology is also suitable to prepare substituted butenolides starting from hydroxy vinyl tellurides (Scheme 14).
Vinylic Tellurides
Vinylic tellurides are important compounds due to their peculiar behaviour as synthons and intermediates. The general preparative procedure is the addition of organyl tellurolates, obtained from reduction of the corresponding ditellurides with NaBH4, to acetylenes. This addition
reac-tion is highly stereoselective, occurring exclusively in anti
mode giving Z adducts41 (Scheme 15), which is in sharp contrast to the well known hydrostannylation42a, hydrozir-conation42b and hydroalumination42c of acetylenes, charac-terized by syn addition modes furnishing E products.42
Z-Divinyl tellurides, in turn, are prepared by addition of Na2Te to activated terminal alkynes41d (Scheme 15). Detellurative cross-coupling reaction with
organocuprates
Substitution of tellurium moiety on vinylic tellurides with retention of the C=C bond geometry has been
per-Ar2TeCl2 or ArTeCl3
Ra-Ni
heat Ar Ar
RTeR1 Pd (0)
heat R R1
Ph 2Te or 2eq. Ph TePh Li2PdCl4 Ph Ph
Scheme 12.
Tol
Ph TePh
+
Pd (II) Tol
Ph+ Tol Ph
Ph TePh + RMgX cat. Ph R
+ Ph Ph
cat. = NiCl2(PPh3)2, CoCl2(PPh3)2, NiCl2.Ph3(CH2)3PPh3
Scheme 13.
Ar TePh CO, PdCl2cat.
CuCl2, Et3N/MeOH
Ar CO2Me
TePh OH
R
R1
CO, PdCl2
Et3N R1 O
R
O
+ PhCO2H
formed with different types of organocuprates. Lithium diorganocuprates43a furnish cross-coupled products with vinylic tellurides (Eq. 3, Scheme 16), and upon reaction with 1,2-ditelluro substituted alkenes, only the terminal tellurium moiety is detellurated. Lower order lithium or magnesium cyanocuprates43b (Eq. 4, Scheme 16) as well as higher order mixed lithium-magnesium, dimagnesium43c
and zinc cyanocuprates43d (Eqs. 5 and 6, Scheme 16) also promote similar reactions. Interestingly, the latter reagents led to complete inversion of the C=C bond geometry in the coupled products.
Olefin Synthesis
Although not so well known as the familiar selenoxide
syn-elimination, the corresponding telluroxide elimina-tion44 has recently gained more attention among the olefin synthesis methodologies. The starting telluroxides are pre-pared by hydrolysis of parent dibromides or by oxidation of tellurides with mCPBA. The elimination reaction, which is preferential towards the less substituted carbon, can occur either at room temperature or upon heating over 200 °C depending on the starting telluroxide. Combination of this methodology with the easy obtention of alkyl phenyl tellurides from the corresponding halides provides a straightforward route to the dehydrobromination of alkyl bromides. Moreover, this methodology can provide vinyl ethers, allyl ethers and alcohols when combined with the alkoxy telluration of alkenes, or with the oxirane ring opening by tellurolates (Scheme 17).
Some noteworthy details have been reported in respect to the elimination reaction, they are: addition of Et3N
increasing the yields of elimination products, pyridinyl tellurium45a moiety as a better leaving group than phenyl
tellurium moiety, and alkyl ferrocenyl tellurides45b being
successfully submitted to elimination sequences in the presence of Et3N or tetracyano ethylene.
Allylic telluroxides undergo [2,3]-sigmatropic rear-rangements46 furnishing allylic alcohols after hydrolysis. The same sequence when performed on chiral allylic fer-rocenyl tellurides shows evidence of chirality transfer to the allylic alcohol as the rearranged product (Scheme 18).
(6) (5) (4) (3)
R = alkyl, vinyl, Ar ; R12 = n-Bu, n-Bu(2-Th)
Ph R1
or R12Cu(CN)(MgBr)2 R12Cu(CN)(MgBr)Li
Ph TeR
R = n-Bu, Ph ; R1 = alkyl, vinyl, Ar
R1
COCF3
R12Cu(CN)(ZnCl)2
RTe COCF3
R2 = prim, sec, tert-alkyl
R = alkyl, Ph, vinyl, alkynyl, CO2Et ; R1 = n-Bu, Ar
R R2
or R2Cu(CN)MgBr R2Cu(CN)Li
R TeR1
Ph Me
Me2CuLi
Ph TePh
Scheme 16.
R Te R
R
Na2Te
EtOH/H2O/THF
NaBH4, NaOH
Te RTeTeR
(a) NaBH4/EtOH
R1 TeR
(b)
R1 = alkyl, aryl, CO2Et, vinyl, alkynyl
R1
Scheme 15.
n
OH
n
OH
Te(O)Ph
n = 2, 3 (b) Br2, - OH
(a) PhTeNa
n
O
n
OMe
n
OMe
Te(O)Ph
n = 1-3 (b) - OH, H2O
(a) PhTeBr3, MeOH
n
R OMe R
OMe
Te(O)Ph
R = n-Hex, Ph (b) - OH, H2O
(a) PhTeBr3, MeOH
R
Scheme 17.
Olefin synthesis via tellurium compounds can also be accomplished by usage of telluronium ylides. They are prepared by treatment of telluronium salts bearing electron-withdrawing groups with bases and can be submitted to Wittig-type olefination reactions with a variety of carbonyl compounds47 (Scheme 19). A high E stereoselectivity is observed in in reactions with aldehydes. These results are especially noteworthy when compared to those from stabi-lized sulfonium ylides which are non reactive towards carbonyl compounds.
On the other hand, semi- and non-stabilized telluronium ylides, for instance di-i-butyl telluronium allylide, di-phenyl telluronium methylide and di-i-butyl telluronium trimethylsilyl propynylide, react with aldehydes and ke-tones giving epoxides48, showing similar chemical beha-viour to non-stabilized sulfonium and selenonium ylides.
Transmetalation Reaction
Te/Li exchange - organolithium reagents
By treatment of diorganyl tellurides with alkyllithiums, a Te/Li exchange reaction takes place generating the most stable organolithium derivative41e,49. This transmetalation is very useful for the preparation of lithium reagents which cannot be achieved by conventional methods such as allyl, benzyl, propargyl, acyl, aroyl and heteroatom substituted methyllithiums. The Te/Li exchange on Z-vinylic tellurides occurs with retention of the original geometry of the C=C bond providing a useful methodology for preparation of
Z-vinyllithiums. The resulting new organolithiums can be trapped with a variety of different electrophiles, as shown in the following representative examples (Scheme 20).
[ O ] = t-BuOOH, NaIO4, H2O2, O2 ; Fc* = ferrocenyl ee = 14-22%
* R R1
OH
H2O
R R1
OTeFc*
[ 2,3 ]
R
R1 TeFc*
O
[ O ]
R
R1 TeFc*
Scheme 18.
Stabilized telluronium ylides
n-Bu2Te
X Y
n-Bu2Te Y ]
X-+ Base
n-Bu2Te Y
R O
R1 R
R1 Y
X = Cl, Br ; Y = CO2Et, COPh, CN, CONHBu-i, Ph ; Base = t-BuOK, NaH/HMPA
- n-Bu2Te
Scheme 19.
E = aldehydes, ketones, PhCOCl, Me3SiCl
E = aldehydes, ketones, Me3SiCl
E = aldehydes, ketones, Me3SiCl
E = aldehydes, ketones, CO2, acid chlorides, RX
Y = PhCH2O, MeO(CH2)2O, Me2N, Me3Si, BuTe, PhSe, Bu3Sn
E
E Y
Li Y n-BuLi
n-BuTe Y
RCOCl
n-BuTeLi
R E
O
E
R Li O
R1Li
R TeBu-n O
E
RE RLi
n-BuTeR
RX
n-BuTeLi n-BuLi n-BuLi
Te
2 R E
(b) 2E (a) 2eq. n-BuLi
R Te R
Ph
Ph OH
PhCHO
Ph Li
n-BuLi
Ph TePh
OH Ph
Ph
PhCHO
PhLi n-BuLi
PhTePr-n
Te/Na, K, Mg, Ca, Zn exchange
Transmetalations with Na, K, Mg and Ca were per-formed on alkyl, alkenyl, aryl, allyl and benzyl tellurides,50 while Te/Zn exchange reaction51 followed by coupling with aryl iodide was achieved under palladium catalysis (Scheme 21).
Te/Cu exchange - organocopper reagents
The most promising synthetic application of Z-vinylic tellurides is their transmetalation with higher order cyanoc-uprates. The counter-ion in the cuprate plays as important role in this reaction. Instead of a detellurative cross-cou-pling reaction as observed with lithium-magnesium and di-magnesium derivatives, the dilithium cyanocuprate pro-motes Te/Cu exchange giving higher order vinylic cyanoc-uprates with retention of the original C=C bond geometry. The resulting Z-vinylic cuprates readily undergo conjugate addition to α,β-unsaturated ketones52a,b, epoxide
ope-ning52b and coupling reaction with bromo alkynes in the presence of ZnCl252c (Scheme 22).
There are some special higher order cyanocuprates such as dimethyl, (2-thienyl, butyl) and (imidazoyl, butyl) dilithium cyanocuprates which carry the so-called ‘‘dummy ligands’’ that are non-transferable from the cuprate, there-fore avoiding formation of undesired addition by-products.
Radical Reaction
By treatment with Li2Te, allylic halides furnishes
1,5-dienes53 via a coupling reaction of the corresponding allylic radicals. Upon irradiation of a mixture containing the acetyl derivative of N-hydroxy-2-thiopyridone (4), alkyl anisyl telluride and an electrophilic olefin, a radical chain mecha-nism54 takes place, which is useful in intramolecular cycli-zations affording six-membered rings (Scheme 23). The same mixture is also useful in tellurium mediated addition of carbohydrates to olefins.
;
R3
R1 R2 ZnCl2
R3 Br
R1
R2 OH
R3 O
R3 O
R1
R2 O
Electrophiles - Products:
R1
R2 E
electrophile
R1
R2 Cu(CN)Li2 R
R2Cu(CN)Li2
R1
R2 TeBu-n
Scheme 22.
4
4 =
4
O M e O
S N
Y = CO2Me, CN
Y
mCPBA
Y S
N Y
hν
p-MeOC6H4Te Y
HO CHO
Scheme 23.
Ph
p-MeC6H4
t-Bu
Pd(PPh3)4
p-MeC6H4I
Ph
EtZn
t-Bu
Et2Zn
Ph
n-BuTe
t-Bu
E = aldehydes, ketones M = Na, K, CaI, MgCl
R = n-Bu, Ph ; R1 = alkyl, alkenyl, allyl, bn, Ar
R1E
E
[ R1M ]
R2M
RTeR1
Scheme 21.
Recently, telluro methyl and methylene substituted tetrahydrofurans55 were obtained from epoxides by
combi-nation of tellurolate promoted oxirane ring opening and radical cyclizations (Scheme 24).
Acknowledgments
The authors acknowledge financial support from FAPESP and CNPq.
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